Northern Soil Carbon Reservoirs Vulnerable to Global Warming

Potential “Hot Zones” for CO2 Emissions and Wildfire

Why Study Soil Carbon? USGS Science at AGU

Mounting evidence shows that soil carbon is increasingly contributing larger amounts of carbon dioxide (CO2) to the atmosphere as a result of warming, permafrost degradation, and complex climate-biogeochemical interactions.

Northern soils are known to harbor large amounts of carbon in the zone between the moss surface and the permafrost or mineral soil boundary. USGS Soil Scientist, Jennifer Harden explains, “When permafrost thaws to create a thicker active layer, microbial processing of previously frozen carbon results in particularly large releases of CO2.” This carbon-rich soil zone is proving be the “hot zone” for CO2 emissions as environmental conditions change through decomposition and wildfires.

Soil is the largest terrestrial reservoir of carbon, containing twice as much as the atmosphere and three times that in global vegetation. It is known to interact with carbon reservoirs in vegetation and the atmosphere by annually transferring significant amounts of carbon. Scientists are researching the role that soil carbon may play in forcing climate change during the coming decades.

Over past centuries northern ecosystems have been very effective at sequestering carbon into soil organic matter, as evidenced by the large stocks of organic carbon in northern soils, particularly permafrost and wetland landforms. USGS scientists and their colleagues are studying soil carbon in the boreal forests of Alaska and Canada, as well as other areas in the United States, to gain insights into global patterns of climate change as well as the effect of disturbances on the soil carbon cycle.

Results of USGS carbon and soil studies will be presented at the American Geophysical Union (AGU) meeting in San Francisco, Dec 15 – 19, in Moscone West Convention Center. USGS scientists will be discussing the large stores of carbon in northern soils, and their vulnerabilities to global change.

The following presentations at AGU will go into depth about these issues:

Organic soils play an important role in boreal ecosystem function by influencing temperature, hydrology, decomposition rates, fire dynamics, and species composition. Research results suggest that soil properties vary by both horizon type and drainage class. A simple model that includes the horizon type directly above the mineral soil, and the organic soil thickness, can be used to accurately predict carbon stocks of organic soil profiles.

Rapid changes in boreal ecosystems resulting from climate change and land surface disturbances from such things as fires and insect outbreaks have triggered substantial changes in ecosystem structure and functions, biogeochemical cycle, and land surface processes. Recognizing that these changes have major implications for regional ecosystem and climate systems, scientists are using a soil physics module to simulate changes of soil temperature, moisture, active layer thickness, permafrost depth, and their impacts on organic layer and soil carbon dynamics at boreal forest sites in the Alaskan Yukon River Basin.

Wildfire can affect the carbon dynamics of high latitude boreal ecosystems directly through combustion emissions and indirectly through vegetation succession and removal of the surface organic layer, which may accelerate the degradation of permafrost — all of which impact the storage and release of soil carbon. In a study of indirect effects, scientists developed and implemented a dynamic soil layer model to investigate the effects of change in the surface organic layer over the Yukon River Basin.

Frozen soils in the arctic and subarctic regions cover a little over 22% of the earth’s land surface and store about 30% of global soil carbon, making the thermally-protected carbon contained within it particularly important to the global carbon cycle. In boreal regions, the carbon contained within active layer and permafrost soil horizons is usually not mineral-protected and incubation studies have revealed that dormant microorganisms can easily metabolize this material as soils thaw.